Why do you assume that they unbuckled their seat belts as opposed to not using them at all? Plenty of people drive without using them.
The fact that this was 4 am makes falling asleep at the wheel more likely, not less.
The fact that both drivers were not using seat belts indicate that they both made impaired decisions.
Were they impaired by alcohol or lack of sleep?
Either way - they paid for their bad choices with their lives.